1. Field of the Invention
The present invention relates to a reducing apparatus for carbon particles. More particularly, the present invention relates to a reducing apparatus for carbon particles which can capture, accumulate, burn and reduce the carbon particles contained in the exhaust gas of a diesel engine.
2. Description of the Prior Art
Exhaust gas from a diesel engine contains fine particles of carbon, i.e., carbon particles PM (particle matter) which are the soot, generated by the incomplete combustion of a fuel.
If such carbon particles PM are directly discharged to the open air, they are harmful to the human body and the environment. It is therefore an important theme to reduce the carbon particles.
<<Prior Art 1>>
FIG. 10 is an explanatory cross-sectional view showing a conventional example of a reducing apparatus for carbon particles PM and the like. As shown in the figure, a catalytic converter 3 is set in an exhaust pipe 2 for exhaust gas 1 discharged from a diesel engine in the prior art. The catalytic converter 3 is provided with a purifier 5 for purifying carbon monoxide CO and hydrocarbon HC, and a reducing apparatus 6 for carbon particles PM in that order within an outer cylindrical casing 4.
In the purifier 5, cell walls 8 of a honeycomb core 7 adhere together and are coated with an oxidation catalyst such as platinum Pt. The purifier 5 oxidizes, burns and reduces carbon monoxide CO and hydrocarbon HC which are harmful substances in the exhaust gas 1, and causes nitrogen oxide NO in the exhaust gas 1 to oxidize into nitrogen dioxide NO2.
In the reducing apparatus 6 for carbon particles PM, pore walls 10 of a high-density porous ceramics-made filter 9 adhere together and are coated with an oxidation catalyst such as platinum Pt. The reducing apparatus 6 is provided to capture, accumulate, oxidize, burn, and reduce the carbon particles PM which are harmful substances in the exhaust gas 1.
<<Prior Art 2>>
FIG. 11 is an explanatory cross-sectional view showing another example of a conventional reducing apparatus for carbon particles PM and the like. A reducing apparatus 11 shown in the figure has been recently developed by an inventor of the present invention, wherein a filter 12 of a wire-mesh structure formed in a substantially column-shape is set within an outer cylindrical casing 4 on the same axis and with the same diameter as the casing 4.
An oxidation catalyst such as platinum Pt is caused to adhere to the wire of the filter 12 and to be coated thereon. The filter 12 captures, accumulates, burns, and reduces the carbon particles PM in the exhaust gas 1 in the same manner as the filter 9 of the reducing apparatus 6 described above.
In the converter 3 of FIG. 11, the purifier 5 also follows the description of FIG. 10.
<<First Problem>>
The following problems have been recognized in such conventional examples. First, a serious problem has been pointed out in that the reducing apparatus 6 for carbon particles PM shown in FIG. 10 is easily caused to break by heating when used.
Namely, in this reducing apparatus 6, after a large amount of carbon particles PM is captured and accumulated by each pore wall 10 of the high-density porous filter 9 made of ceramics, these carbon particles PM are burnt and reduced at one time. In this case, the capture rate and reduction rate of the carbon particles PM amount to 95% or more. Since these carbon particles PM are burnt all together, a sharp rise of temperature of the filter 9 occurs, and the maximum temperature almost reaches 1,200K.
Thus, it is pointed out that the reducing apparatus 6 has a problem in that the filter 9 easily beaks by heating under a high temperature when used.
Namely, once the reducing apparatus 6 and the filter 9 capture, accumulate, burn, and reduce the carbon particles PM, the reducing apparatus 6 and the filter 9 are then regenerated to again capture, accumulate, burn, and reduce new carbon particles PM. In this manner, the reducing apparatus 6 and the filter 9 are expected to repeat a series of cycles of capture, accumulation, burning, and reduction of the carbon particles PM. However, the problem is pointed out that the reducing apparatus 6 and the filter 9 have short lives, are not very durable, and the cost burden is heavy because breakage from heating is easily accelerated to lead to destruction in about a week.
<<Second Problem>>
Second, it has been pointed out that sulfur S contained in the exhaust gas 1 affects accumulation by capture of carbon particles PM and reduction by burning thereof.
Namely, the sulfur S in oil fuel remains in the exhaust gas 1. The sulfur S of a high concentration of about 500 ppm is now contained in the exhaust gas 1 and will be reduced to about 50 ppm. The sulfur S forms sulfate SO42, adheres to the carbon particles PM, or causes clogging in the reducing apparatus 6 for carbon particles PM. As a result, it has been difficult to attain accumulation by capture of the carbon particles PM and reduction by burning thereof in the reducing apparatus 6 and the filter 9.
In the catalytic converter 3, the purifier 5 on the upstream side causes nitrogen oxide NO to oxidize to nitrogen dioxide NO2 which is then supplied to the reducing apparatus 6 on the downstream side. The nitrogen dioxide NO2 has a function of accelerating the burning of the carbon particles PM in the reducing apparatus 6 and helps to solve the problems resulting from the sulfur S described above.
Such a burning acceleration function of nitrogen dioxide NO2 is exhibited at a temperature level of about 600K. However, a sharp temperature rise to about 1,200K is pointed out in the reducing apparatus 6 of FIG. 10 and it has been almost impossible to cause the nitrogen dioxide NO2 to exhibit such a burning acceleration function.
<<Third Problem>>
Third, the reducing apparatus 11 for carbon particles PM shown in FIG. 11 adopts a filter 12 of a wire mesh structure to burn the carbon particles PM in a small quantity before these reach a high temperature. Accordingly, first, it is possible to prevent the breakage by heating described above and second, a bad influence by the sulfur is avoided because the nitrogen dioxide NO2 exhibits the burning acceleration function described above.
However, in this reducing apparatus 11, the filter 12 of a wire mesh structure is set within the outer cylindrical casing 4 on the same axis, and with the same diameter and cross-sectional area as the casing 4. Accordingly, there are problems inasmuch as the intake and exhaust cross-sectional area is small, resistance to the flow of the exhaust gas 1 is large, large resistance is caused by friction and the like, and pressure loss also increases. In particular, these are remarkable every time the carbon particles PM are captured and accumulated by the filter 12.
Serious problems have been pointed out inasmuch as pressure increase of the exhaust gas 1 is caused on the upstream side of the filter 12 and pressure increase within the exhaust pipe 2 is caused on the upstream side, whereby this pressure increase applies excessive load on the diesel engine to increase the driving torque. In the case where the targeted value of the driving torque is, for example, 10 N·m, the actual value has increased to about 12 N·m and as a result, it has a bad influence on the diesel engine.
It has also been pointed out that fuel consumption of the diesel engine deteriorates as the driving torque increases, and the rate of occurrence and the content by percentage of the carbon particles PM in the exhaust gas 1 increase.
<<Fourth Problem>>
Fourth, it is pointed out that there is a problem inasmuch as the reducing apparatus 6 and 11 for carbon particles PM shown in FIGS. 10 and 11 easily generate blow-off.
Namely, the carbon particles PM captured and accumulated by the filters 9 and 12 come off and are blown off in clumps before burning by the exhaust gas 1 linearly passing at a high flow rate. In this manner, the carbon particles PM are easily accumulated on the exhaust pipe 2 on the downstream side or are easily discharged outside.
In particular, in the case where the diesel engine rotates at high speed, much blow-off is generated in accordance with the increase of flow volume and flow rate of the exhaust gas 1.